In oil field applications, it is often necessary to convert energy conveyed on a pressure wave into electrical energy or to generate a pressure wave signal for communication purposes, in both cases under a high background pressure. In such applications, there exists a pressure wave channel, for instance a part of a wellbore filled with fluid, through which energy in the form of a pressure change or pressure wave can be transmitted from one part of the well to other parts of the well. Such pressure waves are often also referred to as acoustic waves.
In a conventional downhole pressure sensor, the sensing element, such as a capsule or a membrane has one side exposed to the pressure to be measured and the other side to a reference pressure, typically a vacuum. The stiffness of the sensing element increases with the pressure range to ensure that the structure does not collapse. The sensitivity of the device is therefore traded off for the pressure range.
For the pressure wave powered downhole electricity generator described in the published international patent application WO 2005/024177 A1, a multilayer piezoelectric ceramic stack, or a solid TERFENOL-D rod, is proposed as the mechanical to electrical energy converter. The main reason for choosing such solid body structures is that they will survive the high-pressure environment.
However, a conversion device based on such a structure can show a poor efficiency in acoustic to electrical energy conversion for several reasons. Firstly, the acoustic impedance of a solid body device is much higher than that of the fluid filled pressure wave channel through which the pressure is applied to the energy converter. Therefore, much of the acoustic energy is reflected away from the fluid/solid interface. Secondly, the strain of the solid body caused by a pressure wave of limited amplitude is very small and thus limiting the magnitude of electrical charge or current generated, which is typically proportional to the strain.
Other devices adapted for a downhole pressure/acoustic wave signal generation are described for example in the published international patent application WO 2005/024182 A1. In that document a pressure wave generator is described based on a multilayer piezoelectric ceramic stack. The generator is suitable for high-pressure environments. However, due to the impedance mismatch between the solid stack and the fluid in the pressure wave channel at the proposed operating frequency, i.e. a few tens Hertz, energy is not always efficiently transmitted into the medium.
A complete system to be used for either downhole power generation or downhole communication will include pressure wave sources that generate the wave from electrical power, and receivers that convert the pressure wave or acoustic energy into an electrical one. An example of a receiver is a pressure wave powered downhole electricity generator as proposed in WO 2005/024177 A1, where the acoustic energy, carried by a low frequency (e.g. 20 Hz) pressure wave generated on surface, is converted into electricity that is used in turn to power downhole electronics.
To produce electricity efficiently from a low frequency pressure wave in a liquid channel, a compliant mechanical structure is needed to convert the pressure first into a strain of sufficient magnitude, which can then be converted into electricity by a strain-to-electricity converter. However in such an application, the downhole steady state pressure is typically in the order of several hundred bars, yet the amplitude of the pressure wave is likely to be in the order of one bar or below. It is therefore a challenge to design a structure that can survive the high background pressure while is still sufficiently compliant to generate the required strain level in response to moderate pressure changes.
There are other examples of converting a small dynamic pressure in a high steady state pressure background. For instance in a conventional measurement-while-drilling operation, mud pulse signals are detected by transducers mounted on a surface stand-pipe. The stand-pipe pressure is typically more than 1000 psi whereas the signal amplitude can be less than 1 psi. Therefore the requirement for the resolution and signal/noise ratio of the detection transducer is very high. In order to withstand the high background pressure, the sensing mechanical membrane of the transducer has to be made sufficiently stiff. The high stiffness, however, can reduce the transducer's sensitivity.
In applications where acoustic communication between downhole devices through the borehole is required, it is essential to have an acoustic source that can deliver sufficient acoustic power at a specified frequency. Since such a source is most likely to be powered by battery or by a downhole energy harvesting system, the efficiency of the source is an important issue.
In systems using for example a sensor plugged into the wall of a borehole some distance away from a cabled section of a well completion as recently proposed, the sensor transmits the measurement data to the cabled section via an acoustic signal. In order to produce a coherent signal for easy detection, it is essential to generate a planar wave propagation mode in the borehole. This means that the carrier wave frequency is preferably low, for example less than 1 kHz.
To generate such a low frequency wave efficiently, a source with sufficiently large cross-sectional area or large displacement is usually needed. A comparison with known sonar transmitters for low frequency underwater communications can show how large such a source would be following conventional designs. At a few kilohertz, the diameter of such a sonar is typically larger than 3 inches [8 cm].
For deployment in the confined space of a borehole, such a large source would be incompatible for many applications including the proposed sensor plug, whose small physical dimensions are its most advantageous feature.
In summary, in the applications discussed above, a dilemma exists between the need for a strong structure to stand high background pressure and that of a compliant one in order to produce sufficient strain. Therefore it remains an object to develop a compact yet efficient downhole sources for sub-kilohertz frequencies.